According to the CIA, "one document outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection. Another memorandum dated June 14, 1918 — written in French — reveals the formula used for German secret ink."

The secrets can now be revealed, CIA Director Leon Panetta says in a statement, because "recent advancements in technology" apparently make the methods obsolete.

— Several pages of research and instructions on secret writing. One tip: "Dip a tooth pick in common milk and write between lines of an ordinary letter. the writing will appear by being ironed out with a hot flatiron."

NPR's Rachel Martin is set to spill more about these secrets on today's edition of All Things Considered. Click here to find an NPR station that broadcasts or streams the show.